Nickel aluminide coatings are known to improve the impact and oxidation resistance of superalloy substrates and their use for this purpose is widely practiced. Although the specific details of the coating processes may vary, the overall process may be briefly described as the formation of a nickel aluminide coating by reacting a source of aluminum with the superalloy substrate. Depending upon the temperatures used, the initial phases formed are delta (Ni.sub.2 Al.sub.3) and beta (NiAl) or almost pure delta. In either case the coated specimen is again heat treated in the absence of aluminum to convert the delta phase to NiAl. This is usually accomplished by a second heat treatment or during high temperature exposure of the aluminized part. In these processes, there is no change in the dimensions of the aluminized part since the coating is formed by inward diffusion of aluminum into the superalloy substrate to form the coating.
Typically, these processes for forming a nickel-aluminide coating on a superalloy substrate involve the packing of the substrate to be coated in an aluminum-containing pack. In addition to the aluminum, other materials can be present, typical of which would be a transfer agent whose function is to assist in the transfer by diffusion of the aluminum from the pack to the substrate and an inert diluent. The pack containing the substrate would be heated up to approximately 2000.degree. F. and maintained at this temperature for a sufficient period of time to enable the aluminum to react with the substrate surface.
It should be recognized that while the overall process is that as described above, specific details can vary such as the environment, i.e., air, vacuum or inert atmosphere; the source of aluminum, i.e., pure aluminum powder or a prealloyed aluminum powder; the type accelerator, if any, i.e., ammonium chloride, ammonium fluoride or other metal halide as well as the temperatures and times of treatment. Numerous references in the prior art exist and U.S. Pat. Nos. 3,647,517 to Milidantri et al, Mar. 7, 1972, for Impact Resistant Coatings for Cobalt-Base Superalloys and the Like; No. 3,764,373 to Speirs et al, Oct. 9, 1973, for Diffusion Coating of Metals are considered representative.
In the practice of this process it has been found that common alloying elements, such as molybdenum and vanadium, for example, which are often found in nickel-base superalloys, exert an adverse effect upon both the oxidation and sulfidation resistance of the aluminide coatings. In order to, in part, counteract this adverse effect, it has been proposed to introduce chromium into the aluminide coating to produce some improvement in oxidation and corrosion resistance. Such approaches are described in U.S. Pat. No. 3,290,126 to Monson, Dec. 6, 1966, for Protectively Coated Nickel or Cobalt Articles and Process of Making and U.S. Pat. No. 3,801,353 to Brill-Edwards, Apr. 2, 1974, for Method for Coating Heat Resistant Alloys. In these processes the nickel-base superalloy substrate is coated with chromium and then the aluminide coating layer is formed over the chromium layer by techniques corresponding to those described above. Since chromium exists in a body-centered cubic crystal structure, and nickel exists in a face-centered cubic crystal structure, the coated articles produced by processes corresponding to those of Monson or Brill-Edwards result in chromium layer between the substrate and the aluminide coating and in the chromium in the aluminide coating existing as a discrete phase. This renders the structure more susceptible to corrosion and thermal attack as a result of the difference in thermal expansion between the body-centered and face-centered cubic structures as well as from the existence of grain boundaries between the Cr and Ni.